Currently, third-generation mobile communication services by means of the code division multiple access (CDMA) scheme have just introduced in wireless (mobile) communication systems, including wireless terminals (user terminals), such as cellular phones. At the same time, next-generation mobile communication technologies capable of providing even faster communications have been under development. The 3rd Generation Partnership Project (3GPP) is studying the Long Term Evolution (LTE), as one of such next-generation mobile communication technologies.
In a mobile communication system, as a preparation to a wireless base station (evolved Node B: eNB) and a user terminal (user equipment: UE) for initiating a communication with each other, a channel is provided for the UE to initiate a transmission to the eNB. The 3GPP refers this channel as a “random access channel (RACH)”, and refers the procedure for initiating a communication through an RACH as a “random access (RA)”.
Random accesses in the LTE are designed in a Slotted Aloha model, wherein a time and frequency resources for sending an RACH are reserved. An RACH contains information, using which the eNB distinguishes UEs from each other which make transmissions. In other words, in order to share an RACH among a plurality of UEs, an identifier called a signature (or preamble) is included.
The respective UEs perform transmission using one signature of a plurality of candidate signatures. Thus, even if the different UEs send signatures via the RACH using the same time and frequency resource, the eNB can distinguish between the UEs based on the received signatures as long as the respective UE employ different signatures.
The RACH is used when initiating a communication, and individual channels (or shared channels) are used afterwards.
A UE make an RA, for example, upon an initial transmission (origination), in response to an incoming transmission from the eNB (generation of downlink (DL) data), upon handover, upon recovery from disconnection (resuming a disconnected communication). As used herein, a wireless link in the direction from the eNB to a UE is referred to as “downlink (DL)”, and a wireless link in the opposite direction as “uplink (UL)”.
There may be some cases wherein an eNB does not recognize some UEs upon an initial transmission or recovery from disconnection, for example, and no individual signatures that can be exclusively used are allocated to the UEs. Such UEs select one of a plurality of (for example, 64) preset signatures for making an RA. Accordingly, a plurality of UEs may simultaneously make RAs using the same signature, although the possibility of such an event is low. Such an RA procedure is referred to as a “contention based random access procedure (contention based RA procedure).
In such a case, the eNB resolves the conflicted signature (select one of the competing UEs), and sends a response to the selected UE. Each UE determines that UE is selected by the eNB whether a response is received from the eNB. The UE selected by the eNB continues the communication (RA procedure) with the eNB, makes settings for the wireless channel with the eNB, and so on. The UE that was not selected by the eNB tries to retry an RA after a predetermined time interval or otherwise.
Note that such a contention of a signature occurs while a UE makes a handover to switch between eNBs for connecting, the connection might be temporarily disconnected or sometimes a communication might be disconnected. For this purpose, the LTE proposes to allocate individual signatures in advance to each of UEs for making a handover. Such an RA procedure is referred to as “non-contention based random access procedure (non-contention based RA procedure)”.    Non-Patent Document 1: 3GPP TS 36.321 V8.1.0; “Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification”, searched online on May 22, 2008, on the Internet; URL: http://www.3gpp.org/ftp/Specs/html-info/36321.htm